Studies of polysaccharide adsorption onto model cellulose surfaces and self-assembled monolayers by surface plasmon resonance spectroscopy
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Throughout the study of polymer adsorption at the air/water and solid/water interfaces, surface tension measurements and surface plasmon resonance (SPR) spectroscopy have been identified as key methods for the acquisition of structural and thermodynamic information. These techniques were used to determine air/water and cellulose/water interfacial properties of pullulan (P) and pullulan cinnamates (PCs), 2-hydroxypropyltrimethylammonium xylans (HPMAXs), and hydroxypropyl xylans (HPXs).
Hydrophobic modification of pullulan with cinnamate groups promoted adsorption onto model surfaces of regenerated cellulose. In order to understand the relative contributions of hydrophilic and hydrophobic interactions towards PC adsorption, PC adsorption onto self-assembled monolayers (SAMs) with different functional groups was also studied. As the degree of cinnamate substitution increased, greater adsorption onto cellulose, methyl-terminated SAMs (SAM-CH3), and hydroxyl-terminated SAMs (SAM-OH) was observed. This study showed that hydrogen bonding alone could not provide a complete explanation for PC adsorption onto cellulose.
The adsorption of cationic 2-hydroxypropyltrimethylammonium (HPMA) xylans with different degrees of substitution (DS) onto SAMs and regenerated cellulose was studied by SPR. Surface concentration (Ð ) exhibited a maximum (Ð max) for HPMAX adsorption onto carboxylic acid-terminated SAMs (SAM-COOH) at an intermediate HPMA DS of 0.10. This observation was indicative of a relatively flat conformation for adsorbed HPMAXs with higher HPMA DS because of higher linear charge densities along the polymer backbone. Ð observed for HPMAX adsorption onto regenerated cellulose and SAM-OH surfaces was relatively low compared to HPMAX adsorption onto SAM-COOH surfaces.
Surface tension measurements for aqueous solutions of HPX by the Wilhelmy plate technique showed that surface tension changes (â Î³ = Î³water â Î³HPX(aq)) increased and critical aggregation concentrations generally decreased with increasing hydroxypropyl (HP) DS. Hence, even though HP substitution was necessary to induce aqueous solubility, excessive hydroxypropylation promoted aggregation in water. SPR studies indicated that HPXs did not adsorb significantly onto regenerated cellulose or SAM-OH surfaces (submonolayer coverage). In contrast, HPX did adsorb (~monolayer coverage) onto SAM-CH3 surfaces.
Collectively, these studies showed natural polymers could be chemically modified to produce surface modifying agents with sufficient chemical control, whereby the surface properties of the resulting systems could be explained in terms of chemical structure and intermolecular interactions.
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